The polyethylene resin expansion molded articles are widely used for buffer packing materials or the like. These polyethylene resin expansion molded articles are produced from polyethylene resin expanded particles.
In a case where a polyethylene resin expansion molded article is produced from polyethylene resin expanded particles, thus obtained expansion molded article has a large dimensional shrinkage factor with respect to a metal mold and a poor dimensional accuracy. This problem tends to emerge noticeably particularly in a case where the polyethylene resin expanded particles are obtained by using inorganic gas as a foaming agent.
In order to solve the above problem, various studies have been made. For example, studies have been carried out on a method in which foaming capability is given by adding various molding gases to expanded particles (Patent Literatures 1 and 2), a method in which foaming capability is provided by compression of expanded particles (Patent Literature 3), a method in which a ratio of an expansion ratio of pre-expanded particles and an expansion ratio of expanded particles used in molding are controlled within a predetermined range (Patent Literature 4), and a method in which a complex temperature history is given after molding (Patent Literatures 5 and 6).
However, each of the above methods has advantages and disadvantages. Further, there are restriction due to equipment and many other problems that need to be solved. For example, in the case where molding gas is added as described in Patent Literatures 1 or 2, equipment for adding gas to expanded particles is required. Further, for preventing deterioration in given foaming capability, such equipment for adding gas and a molding process need to be integrated or arranged to be continuous.
For the method in which expanded particles are compressed as described in Patent Literature 3, equipment for the compression is required. Further, such equipment for the compression needs to be integrated with a molding machine. In addition, in this method, there are problems such that performance in filling the expanded particles is poor and a ratio at which defective products occur is high as compared to a case where the expanded particles are not compressed.
In regard to the method as described in Patent Literature 4 in which a ratio of an expansion ratio of pre-expanded particles and an expansion ratio of expanded particles used in molding is controlled, it is necessary to prepare pre-expanded particles having an expansion ratio that is unnecessarily high. Further, for controlling the ratio of the expansion ratios described above, it is necessary to retain the pre-expanded particles in an atmosphere at a temperature of approximately 60° C. for hours so that the expansion ratio of the pre-expanded particles is adjusted to a desired level.
In regard to the method as described in Patent Literatures 5 and 6 in which a complex temperature history is given after molding, work operations are complicated. Therefore, this method is not advantageous as a production process.
As described above, it has been difficult to easily obtain a noncrosslinked polyethylene resin expanded particles that allows providing, by a simple method, a polyethylene resin expansion molded article whose dimensional shrinkage factor with respect to a metal mold is small, whose deformation is small, and whose surface stretches well.